This is rather broad. The only answer we can give you at this point is to play the song over and over again while improvising keyboard parts, and then write down one that sounds good. Can you be more specific about what exactly you are asking, or with more contextual information about what you're working with?
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NReilingh♦Aug 27 '12 at 15:37

When I first start trying to write something, I might just play each chord on every beat, or once per measure to get a feel for it. Then I think about which inversions sound good, and how one leads to the next. I also think about how to vary it rhythmically.

Usually I have an idea of what sort of sounds I want to use. In your case, you're already thinking of organ, so I'd start with that. I might also try something that's completely the opposite, just to see how it sounds and give me new ideas. Sometimes I'll record something into the sequencer and just switch patches as it plays back to see what it sounds like, but usually I have some direction I try to take it.

And of course, you don't want to upstage the main performer. Is the piece written to showcase a singer? Guitar? Something else? I usually try to stay out of the range of the lead so as to not overwhelm it.

You need to listen to the song and decide the; tone, chords and key of the song, then make sure that you stick to these three things. using similar complementary notes and similar chords (maybe a harmony of the song) will make the song have more death and a richer tone. Basically you need to listen and see what the song already sounds like and where there are gaps. This is a tricky question and is not that simple because what i would do to a song is very different to what you would do.

That's kind of a hard question to answer and I am assuming the answer is very different for everyone. When I am witting a part for a song I pull out a tape recorder and just play it and see what sounds good.

Hi John, welcome to the site. Short answers like this are likely to get down voted. If you want to improve the answer, you could add more detail about your past experiences - what has worked well, what hasn't, and in what situations. Adding something that is not already included in other answers is especially useful.
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naught101Sep 16 '12 at 7:28